User equipment (UE), also known as mobile stations, wireless terminals and/or mobile terminals are enabled to communicate wirelessly in a wireless communication system, sometimes also referred to as a cellular radio system. The communication may be made e.g. between two user equipment units, between a user equipment and a regular telephone and/or between a user equipment and a server via a Radio Access Network (RAN) and possibly one or more core networks.
The user equipment units may further be referred to as mobile telephones, cellular telephones, laptops with wireless capability. The user equipment units in the present context may be, for example, portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile devices, enabled to communicate voice and/or data, via the radio access network, with another entity, such as another user equipment or a server.
The wireless communication system covers a geographical area which is divided into cell areas, with each cell area being served by a network node, or base station e.g. a Radio Base Station (RBS), which in some networks may be referred to as “eNB”, “eNodeB”, “NodeB” or “B node”, depending on the technology and terminology used. The network nodes may be of different classes such as e.g. macro eNodeB, home eNodeB or pico base station, based on transmission power and thereby also cell size. A cell is the geographical area where radio coverage is provided by the network node/base station at a base station site. One base station, situated on the base station site, may serve one or several cells. The network nodes communicate over the air interface operating on radio frequencies with the user equipment units within range of the respective network node.
In some radio access networks, several network nodes may be connected, e.g. by landlines or microwave, to a Radio Network Controller (RNC) e.g. in Universal Mobile Telecommunications System (UMTS). The RNC, also sometimes termed a Base Station Controller (BSC) e.g. in GSM, may supervise and coordinate various activities of the plural network nodes connected thereto. GSM is an abbreviation for Global System for Mobile Communications (originally: Groupe Spécial Mobile).
In 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE), network nodes, or base stations, which may be referred to as eNodeBs or eNBs, may be connected via a gateway e.g. a radio access gateway, to one or more core networks.
The 3GPP is responsible for the standardization of LTE. LTE is a technology for realizing high-speed packet-based communication that may reach high data rates both in the downlink and in the uplink, and is thought of as a next generation mobile communication system relative UMTS.
Relaying is being considered for LTE Rel-10 as a tool to improve the coverage of high data rates, group mobility, temporary network deployment, the cell-edge throughput and/or to provide coverage in new areas. A relevant aspect of relaying networks is the spectrum resources used for the backhaul. One distinguishes between out-of-band and in-band relaying. In the former case, the backhaul link uses a different frequency carrier and even a different access technology than the access link uses. In this case, when using out-of-band relaying, the interference between the two links may be negligible and the two links may be managed independent of each other. In the latter case, in in-band relaying, the backhaul and the access link use the same carrier and therefore may interfere with each other.
Towards the base station, the relay node acts as a user equipment. Towards the user equipment, the relay node acts as a base station. The user equipment may not make any logical distinction between a cell created by a relay node and the cell created by a regular base station. In fact the user equipment may not even be aware of the existence of the radio backhaul connection.
A typical limitation of in-band relays is thus that they are not able to send and receive on the same channel, at the same time, i.e., they use a half-duplex communication mode. Therefore in-band-relaying may lead to increased interference and increased data- buffer lengths, causing increased delays within the network.
A type 1 relay node, which may be part of LTE-Advanced, is an in-band relay node where the backhaul link, i.e. the link between the donor base station and the relay node, and the access link, i.e. between the relay node and the user equipment, share the same spectrum.
To avoid self-interference at the relay node, and at the same time maintain backward compatibility to LTE Rel-8 user equipment, Multi-Media Broadcast over a Single Frequency Network (MBSFN) subframes are configured for the access link to create downlink transmission gaps for eNB-to-relay transmissions, as illustrated in FIG. 1. For uplink, the transmission gap is created by means of scheduling restriction.
In the present context, the expressions downlink, downstream link or forward link may be used for the transmission path from the network node, possibly via a relay node, to the user equipment. The expression uplink, upstream link or reverse link may be used for the transmission path in the opposite direction i.e. from the user equipment, possibly via a relay node, to the network node.
FIG. 1 illustrates an example of relay-to-UE communication using normal subframes (left) and eNodeB-to-relay communication using MBSFN subframes.
In an MBSFN subframe, a relay node first transmits control signal to its subordinate user equipment, then switch to receiving mode to receive data (Tx-Rx) from donor eNB, and then switch to transmitting mode again (Rx-Tx). For uplink, the Tx-Rx switch and Rx-Tx switch are also needed. Due to implementation restrictions in the radio unit, some time is needed for the switch from Tx to Rx and from Rx to Tx, which may be referred to as switch time.
For different length of switch time, the timing relationship between backhaul and access link may be different, which has between discussed in 3GPP. Assume the relay node may receive Un downlink transmissions starting with Orthogonal Frequency-Division Multiplexing (OFDM) symbol numbered m and it may stop receiving with the OFDM symbol numbered n, and k is equal to the number of OFDM symbols used for the L1/L2 control region at the relay node access. For downlink, the following two cases may be considered:
Case 1: the relay node may receive the downlink backhaul subframe starting from OFDM symbol m=k+1 until the end of the subframe (n=13 in case of normal cyclic prefix). This corresponds to the case when the relay node switching time is longer (>cyclic prefix) and relay node downlink access transmit time is slightly offset with respect to downlink backhaul reception time at the relay node.
Case 2: the relay node may receive the downlink backhaul subframe starting from OFDM symbol m=k until the end of the subframe (n=13 in case of normal cyclic prefix). This corresponds to the case when the relay node switching time is sufficiently shorter than the cyclic prefix and relay node downlink access transmit time is aligned to the downlink backhaul reception time at the relay node.
For uplink, similar timing relationships may comprise:
Case 1: the relay node may transmit Single-Carrier Frequency-Division Multiple Access (SC-FDMA) symbols m=1 until the end of the uplink backhaul subframe (n=13 in case of normal cyclic prefix). This corresponds to the case when the access link and backhaul link uplink subframe boundary is staggered by a fixed gap.
Case 2: the relay node may transmit SC-FDMA symbols m=0 until the end of the uplink backhaul subframe (n=13 in case of normal cyclic prefix).
This corresponds to the case when the access link uplink subframe boundary is aligned with the backhaul link uplink subframe boundary and relay node switching time is sufficiently shorter than the cyclic prefix (case 2a).
Alternatively, this corresponds to the case when the access link and backhaul link uplink subframe boundary is staggered by a fixed gap and relay node switching time is considered by configuring the user equipment not to transmit the last SC-FDMA symbol of the Uu link (case 2b).
3GPP work focuses mainly on cases that the switch time of the relay node is larger than the cyclic prefix. For switch time less than the cyclic prefix, no solution is provided. When the relay node switch time is less than the cyclic prefix (4.7 ps in cases of normal cyclic prefix), there is no OFDM symbol reserved for Tx-Rx and Rx-Tx switch. In such case, accurate timing alignment between access link and backhaul link is needed, otherwise interference will be introduced. This requirement is even stricter when a Main Unit-Remote Radio Unit (MU-RRU) architecture is used for the relay node and the relay node's backhaul side antenna and access side antenna are situated far apart from each other.
As the subsequent disclosure exclusively relates to half-duplex in-band relay nodes, the term in-band is omitted in the subsequent text. By “relaying” it is herein meant “in-band relaying”, unless otherwise stated.
A common problem when relaying in half-duplex is that effective data-rate performance for some user equipment may actually be degraded when using relay nodes.
However, the limitation of half duplex transmission over the backhaul link also leads to a situation where the end-to-end data rate performance decrease when transmitting via the relay node, in comparison to direct transmission from the base station, as the resources have to be shared between the backhaul link and the link between the relay node and the user equipment. There may also be a delay in the relay node when shifting from receiving signals from the base station over the backhaul link and transmitting signals to the user equipment within the own cell, leading to a degradation of performance within the wireless communication system.
Thus there is a desire for improvements in communication systems comprising relay nodes.